undef reg in gdb_curses.h

undef reg in gdb_curses.h

I tried to build gdb on the AIX machine in the GCC compile farm
(gcc111), but it failed in a couple of spots because gdb uses "reg" as
a variable name and the AIX <curses.h> defines "reg" to "register".

I saw that we already had a workaround for this lurking in utils.c, so
I just moved that to gdb_curses.h.

This fixed the problem on AIX and still builds on x86-64 Fedora 18.

2013-10-29  Tom Tromey  <tromey@redhat.com>

	* utils.c (reg): Move undefinition...
	* gdb_curses.h: ... here.  Update comment to mention AIX.